The present invention relates to a method for treating waste water discharged from catalytic cracking units at an oil refinery.
Catalytic cracking is chiefly performed to produce gasoline of an increased octane number by cracking, in the presence of a catalyst, those fractions which have higher boiling points than kerosene. Catalytic cracking of petroleum has conventionally been achieved with various types of reactors, including a fluidized-bed unit, a moving-bed unit, and a fixed-bed unit. Recently, techniques have been developed either an atmospheric or vacuum residual oil be mixed in the feed to the catalytic cracking unit.
In the operation of a catalytic cracking unit as described in Direction for Industrial Waste Processing Technique, published by Public Pollution Safety Board of Japanese Ministry of International Trade and Industry in March, 1973, steam is injected at various sites within the unit in order to achieve such objects as preventing carbonization, assisting in oil-catalyst separation, dispersing the catalyst, and achieving oil stripping from the catalyst. The steam fed emerges from the top of a reactor together with the decomposition product. The decomposition product is subjected to purification in a fractionating tower and a stripper, which are also fed with stripping steam in order to achieve enhanced fractionation. All of the steam came to the top of the fractionating tower together with light oils, and is cooled, and separated from the light oils in a condenser, and is discharged from the system as waste water. The principal components of the waste water from the catalytic cracking unit are shown in Table 1, from which one will be be able to see that it contains large amounts of ammonia, hydrogen sulfide and phenols, as well as having a high COD (chemical oxygen demand). Therefore, of all the waste waters originating from an oil refinery, the one discharged from a catalytic cracking unit is most refractory to efficient treatment.
TABLE 1 ______________________________________ pH 8-9.5 oils 1-10 ppm NH.sub.4 1,000-7,000 ppm S.sup.2- 100-1,000 ppm phenols* 10-1,000 ppm COD 300-2,700 ppm CN.sup.- 20-60 ppm ______________________________________ *Phenols including phenol, o, m, pcresol, etc.
As described at pages 58-63 in Circumstance Creation, Activated Carbon Treatment for Waste Water from Petroleum Refinery or Petro-Chemical Factory, published in 1975, the conventional method for treating waste water discharged from a catalytic cracking unit generally proceeds as follows: hydrogen sulfide and ammonia are first stripped out of the waste water, which is then passed through a crude oil desalting unit or a phenol extracting unit to extract part of the phenols; subsequently the waste water is diluted with other process waste waters or industrial water; the diluted waste water is treated by the continuous activated sludge process at a pH in the neutral range; finally, in order to meet standards of the emissions of COD and phenols, the activated sludge treated waste water is subjected to treatment with activated carbon.
The waste water to be treated by the aforementioned method contains phenols, hydrogen sulfide, and other substances that are harmful to microorganisms. In particular, phenols cannot be removed satisfactorily even by extraction with crude oil or other solvents. Therefore, sludge bulking will readily occur during continuous activated sludge treatment of the undiluted or slightly diluted waste water. In operation of the activated-sludge plant, utmost care should be exercised to hold the sludge volume index (SVI) at no greater than 100 ml/g. The reason therefor is that should sludge bulking occur to cause an increase in the SVI, further treatment of the waste water would become impossible. In addition, municipal governments have set fairly strict emission standards regulating COD and phenol levels in waste waters from an oil refinery.
In order to meet these conditions, commercial catalytic cracking units operating today employ combinations of activated sludge treatment of highly diluted waste water and subsequent treatment with activated carbon. Therefore, efficient treatment of waste water from a catalytic cracking unit requires contaminants such as COD and phenols to be sufficiently removed in the step of activated sludge treatment so as to minimize the loading of contaminants for the subsequent step of treatment with activated carbon.
The above described SVI, which is also referred to as "Mohlman Index", is represented by: ##EQU1## More specifically, SVI is defined as the volume in milliliters occupied by one gram of sludge in a mixed-liquor sample (taken at the outlet of the aeration tank) after settling for 30 min in a 1000-ml graduated cylinder.
Throughout this specification, "catalytic petroleum cracking unit" implies an apparatus for decomposing hydrocarbons such as petroleum having a boiling point higher than that of kerosene and aromatic compounds using a catalyst.